1. The leaves fallen on the ground were wet and dirty.

2. The leaves which had been fallen on the ground were wet and dirty.

3. The leaves which was fallen on the ground were wet and dirty.

(Does #1 mean #2 or #3?)

No, sorry.

Read for corrections:

1. The leaves on the ground were wet and dirty.

2. The leaves which had fallen on the ground were wet and dirty.

3. The leaves which fell on the ground were wet and dirty.